Deep levels criteria

The next step is the definition of deep. The choice of quantitative values will again involve some arbitrariness. However, a further complication is that there is at present no universally accepted qualitative criterion. For instance, from the point of view of energy level calculations it is often convenient to define deep states as noneffective-mass-like or as those with a localized potential (see, for example, Bassani and Pastori Parravicini, 1975 Jaros, 1980). However, the disadvantage of this definition of deep is that it includes many isoelectronic states that are very shallow on an energy scale. On the other hand, if one uses an energy criterion, should the states be deep with respect to some fraction of the band gap, with respect to kT, or with respect to some shallow levels In this chapter we shall adopt an energy criterion for deep, and we shall require that our states be deep enough to be important in recombination. The importance of deep levels in recombination under many conditions of practical interest was already realized in the early work of Hall... 

Adoption of the concept of kinetic criteria allows a simple optimization of protective installation working parameters. In many cases it is unnecessary to fully retard the corrosion of a protected metal structure. Deep cathodic polarization is an expensive process. It requires the application of high power equipment, developed anodic systems, and is connected with high operation costs. For practical reasons, partial protection is sufficient in most cases, ensuring a decrease of the corrosion rate to such a level at which the assumed lifetime of the structure is attained. Evaluation of the effectiveness of cathodic protection based on a criterion taking into account the degree of decrease of corrosion process rates can become in the near future a new, important application element in the anticorrosion protection technology. 

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